Received on July 13, 2004
Accepted on February 1, 2005

Proteomics and Protein Markers

Comparison of Cardiac Troponin I Immunoassays Variably Affected by Circulating Autoantibodies

¹ Departments of Biotechnology and Medicine, University of Turku, Turku, Finland, and Biotechnology
² Departments of Biotechnology and Medicine, University of Turku, Turku, Finland, and Medicine, University of Turku, Turku, Finland
³ Departments of Biotechnology and Medicine, University of Turku, Turku, Finland, and Department of Laboratory Medicine, Division of Clinical Chemistry, Malmö University Hospital, Malmö, Sweden
⁴ Departments of Biotechnology and Medicine, University of Turku, Turku, Finland, and Laboratory Diagnostics
⁵ Departments of Biotechnology and Medicine, University of Turku, Turku, Finland, and Medicine, Helsinki University Central Hospital, Helsinki, Finland

^* To whom correspondence should be addressed. E-mail: susann.eriksson{at}utu.fi.

Background: We recently provided evidence that circulating autoantibodies against cardiac troponin I (cTnI) or the troponin complex cause negative interference in cTnI immunoassays. By comparing three cTnI immunoassays, we further explored the phenomenon of circulating autoantibodies and their consequences in patient samples.

Methods: We developed a cTnI immunoassay with a novel assay design using three antibodies, two of which bind epitopes outside the stable, central part of cTnI. Samples from 541 chest pain patients were measured with the new cTnI assay and with a first-generation cTnI assay (Innotrac Aio cTnI) using a conventional midfragment assay design. Using another sample cohort, we also compared the new assay with a second-generation cTnI assay (Access AccuTnI).

Results: The analytical detection limit of the new cTnI assay was 0.012 µg/L, and the lowest concentration giving a total imprecision (CV) of 10% was 0.060 µg/L. The mean difference (95% confidence interval) between the new cTnI and Aio cTnI assays was larger in admission samples (21.0%; -107.8% to 149.7%) than in samples taken 6-12 h 12.8%; -61.5% to 87.2%) and 24 h after admission (3.0%; -71.3% to 77.4%; P <0.001). With the lowest concentrations giving 10% CV (0.22 µg/L for Aio cTnI) used as cutoffs, 14.3% (n = 76) of admission samples were positive only with the new assay, whereas 13.5% (n = 72) were positive with both assays. Of samples taken at 6-12 and 24 h, 10.2% (n = 31) and 8.3% (n = 29) were positive only with the new assay. ROC curve analysis of admission samples showed a significantly higher area under the curve for the new cTnI assay (0.940) than for the Aio cTnI assay (0.846; P <0.001). The new cTnI assay gave generally lower results than the AccuTnI assay; the mean (95% confidence intervals) differences were -58.9% (-151.8% to 34.0%) in admission samples. In samples with severe interference from autoantibodies, median ratios between the new assay and AccuTnI were higher than in samples with no apparent troponin autoantibodies (0.875 vs 0.470; P<0.001).

Conclusions: The new cTnI assay, which is based on a novel antibody combination different from the conventional midfragment antibody approach, offers improved detection of cTnI in samples containing troponin autoantibodies.